1. Field of the Invention
The present invention relates to oil-based muds or fluids for use in drilling wellbores in subterranean formations. More particularly, the invention relates to oil-based muds suitable or compatible for use with resistivity imaging of the wellbore or the formation.
2. Description of Relevant Art
In the oil and gas industry, there are generally three primary or commonly used techniques for imaging of wellbores. These three techniques are: acoustic imaging, azimuthal density neutron imaging, and resistivity imaging. Of the three, resistivity imaging is often preferred for its relative simplicity of operation, rapid acquisition of real-time data, and highly accurate images of geological features.
Resistivity imaging techniques are based on measurements of the electrical resistance of the formation exposed to the wellbore. Tools based on this technique, such as Halliburton's Electro-Micro-Imager™ (EMI), available from Halliburton Energy Services, Inc. in Houston, Tex., typically apply an alternating current of approximately one volt at an operating frequency of about 15 kHz. The current is applied to the formation through a series of pads that are pressed firmly against the wellbore. After passing into the formation, the signal is conducted back to a detector on the tool by the wellbore fluid.
The electrical conductivity of a wellbore fluid directly affects the quality of the resolved image. If the fluid is overly conductive, such as a concentrated salt brine, a poorer image may result due to an electrical short preventing good electrical penetration into the formation. If the fluid behaves as a dielectric or a capacitor, the lack of a signal or an unfavorable signal-to-noise ratio may prevent satisfactory image resolution. Such dielectric behavior is exhibited by a traditional oil-based mud. The continuous hydrocarbon phase of an oil mud is an electrical insulator, causing the oil mud to behave in a dielectric manner. Thus, resistance imaging devices are typically used when the wellbore contains an aqueous fluid, or when aqueous-based drilling fluids are used to drill the wellbore.
In resistance logging applications, the EMI tool is serviceable in fluids having resistances of about 2000 to about 0.2 ohm-m. Preferably, the fluid has a minimum conductivity in the range of about 1–10 ms/m (millisiemens-per-meter), which is equivalent to about 10- to about 100 μs/cm. These values are represented in terms of resistivity by about 1000- to about 100 ohm-m. The serviceability of any conductive fluid is dependent on its lack of dielectric (electrically insulating) properties. A fluid that exhibits low conductivity and no dielectric or capacitance properties should, at least theoretically, be capable of yielding or facilitating good wellbore images, owing to its favorable signal-to-noise ratio.
U.S. Pat. No. 6,006,831, issued Dec. 28, 1999 and assigned to Schlumberger Technology Corp., discloses additives comprising certain metallic and non-metallic particles and fibrous materials said to enhance the images obtainable with electrical logging techniques when added to certain wellbore fluids. The wellbore fluids are either based on traditional invert emulsion drilling fluid technology or are 100% oil fluids. These additives have an aspect ratio of two or greater. They are said to improve electric logging results by bridging solely among themselves, by linking the emulsified brine droplets, or by a combination of these two mechanisms. In the patent, most of the treated fluids were said to display electrical stabilities well above 100 volts; the lowest electrical stabilities were in the 15–20 volt range. However, these fluids showed relatively high resistivities, making them unfavorable for most drilling or well service fluid applications.
International patent application publication number WO 00/41480, published Jul. 20, 2000 and assigned to Sofitech N.V., Schlumberger Canada Limited, and Dowell Schlumberger, discloses invert emulsion drilling fluids containing carbon black particles having preferred surface areas of 500–1500 m2/g at concentrations of 0.2–10% by volume. Although, traditional calcium fatty soap-related technologies used to prepare invert emulsions are said to tend to modify the surface of carbon black and render it nonconductive in such applications, monovalent surfactants, such as sodium ion fatty acid soaps, or nonionic surfactants, used to prepare invert emulsions, are said not to modify the carbon black. Rather, the carbon black particles in such emulsions are said to be free to associate such that conductivity through the fluid is established. Examples of surfactant chemistries said to be preferred for the invention are diethanolamides, alcohol alkoxylates, copolymers, fatty acids, phosphate esters, and phosphonates. The internal brine phase may contain monovalent salts and any materials that complex with or precipitate polyvalent ions. Conductivity near the 100 μs/cm level is cited in an example. Applications claimed include drilling, logging, measurement-while-drilling (MWD), and logging-while-drilling (LWD).
Notwithstanding laboratory results reported in the WO/00/41480 publication, fluids based on traditional albeit monovalent or nonionic surfactant invert oil emulsions with electrically conductive properties imparted to them by conductive particle additives are still expected to possess traditional invert emulsion drilling fluid properties. Such fluids will not necessarily have the electrically conductive properties of a continuous aqueous phase. The dielectric (electrically insulating) properties of the oil will undoubtedly have an effect in the field on the conductivity of the particles, causing such fluids to display conductivity having dielectric or capacitance properties, which in turn can result in the detected logging current having a poor signal-to-noise ratio. Even where conductive properties seem favorable in lab tests, any source of multivalent cations in the actual wellbore, such as may be caused by lime or salt water flows in the field, can affect the conductive properties and result in a poor signal to noise ratio. Even without disruption of conductivity in the whole drilling fluid, filtrate that invades the formation may exhibit little or no conductivity.
U.S. Pat. No. 6,029,755, issued Feb. 29, 2000 and assigned to M-I L.L.C., discloses a fluid composition of an oleaginous fluid, an aqueous solution containing electrolytic salts, an emulsifier, and a water-soluble alcohol said to be useful for resistivity imaging. However, the fluid prepared according to the patent is actually not believed to be a true invert emulsion oil mud. The oleaginous fluid is said to preferably be oil-soluble glycols, butyl ethers of propylene glycols, or a mixture of these with synthetic, mineral, vegetable, or silicone oils. These glycol-based chemistries represent the well-known class of materials known as mutual solvents. The aqueous solution is said to contain any of the following electrolytic salts: chlorides of sodium, potassium, ammonium, magnesium, and calcium; bromides of sodium and calcium; acetates of sodium and potassium; or citrate of sodium. The emulsifier is said to consist of tall oil fatty acid (TOFA), oxidized TOFA, surfactants, phosphate esters, amidoamines, or imidazolines. The water-soluble alcohol component is said to consist of ethylene glycols, propylene glycols, ethylene oxide/propylene oxide (EO/PO) copolymers, or butylene glycol (1,4-butane diol). Supplemental additives described include clay-based or polymeric gelling agents and corrosion inhibitors. Conductivity in such fluid is believed to be occurring through the aqueous phase. Because this fluid is not believed to be a true invert emulsion oil mud, it is not believed capable of preventing shale dispersion or inhibition in the manner of a traditional invert emulsion drilling fluid. Further, mutual solvents are well known to have detrimental effects on polymeric components of rig equipment.
There continues to be a need for new and improved oil-based invert emulsion drilling fluids with electrical conductive properties suitable for use with resistivity logging.